Starlionblue From Greenland, joined Feb 2004, 17039 posts, RR: 66 Posted (10 years 1 month 1 week 5 days 7 hours ago) and read 32767 times:
Just what the topic says. I know that engines today are axial flow, while engines in the dark ages (1930-50?) were centrifugal flow.
I wondered how exactly this is defined and what it means, because the authors of all my aviation books and lots of websites seem to take the knowledge for granted. I know how jet engines work, and that annular combustors have replaced cans, but this does not answer the question.
"There are no stupid questions, but there are a lot of inquisitive idiots."
Airplay From , joined Dec 1969, posts, RR:
Reply 1, posted (10 years 1 month 1 week 5 days 7 hours ago) and read 32767 times:
There are two types of compressors in use in modern jet engines. Axial flow compressors move the air into ever smaller spaces using a series of axial compressor fans. Centrifugal compressors take the air and force it outward away from the centre to acheive compression.
Therefore the overal flow for axial flow engines is in a straight line, whereas the airflow through centrifugal engines changes directions through each stage.
Centrifugal compressors are not ancient or outdated, except when you consider some of the larger modern jet engines.
The successful (and still in production) PT-6 line uses centrifugal compressors. Also, some engines use a combination of centrifugal and axial stages.
411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 7, posted (10 years 1 month 1 week 5 days 5 hours ago) and read 32767 times:
Centrifugal compressors are certinally nothing new to large piston engine as well.
ALL of the older large radial piston engines used centrifugal design superchargers (not turbochargers) to enhance power.
The fuel was routed from the pressure carbueretor to the center of the supercharger, where it was mixed with air and pressurized in the intake manifold, and delivered to cylinders.
With some engines, 59+ inches of manifold pressure was achieved for takeoff.
Many of these same engines had two speed superchargers.
At approximately 12,000msl, the superchargers (blowers) were shifted, thru a conical clutch arrangement, to a different gear drive ratio, in order to spin faster...for enhanced high altitude ops.
A very few large piston engines used both superchargers AND turbochargers.
One was the Pratt&Whitney R4360, and could be successfully operated up to 40,000 feet.
These particular engines were fitted to the Boeing Stratocruiser.
Kalakaua From United States of America, joined Jan 2004, 1516 posts, RR: 5
Reply 8, posted (10 years 1 month 1 week 5 days 5 hours ago) and read 32767 times:
The centrifugal flow compresor has a single or two stage unit using an impeller to accelerate the air and a diffuser to produce the required pressure rise. The axial flow compressor is a multi-stage unit using alternate rows of rotating (rotor) blades and stationary (stator) vanes, to accelerate and diffuse the air until the required pressure rise is reached. Particularly on small engines, an axial compresor is used to boost the inlet pressure to the centrifugal.
The centrifugal compressor is more rugged than the axial and is also easier to develop and manufacture. But the axial compresor consumes more air than a centrifugal compressor even with the same frontal area. Axials also can be designed to attain much higher pressure ratios.
Air flow is an important factor because it determines the amount of thrust. Axial compressor engine gives more thrust for the same frontal area of a centrifugal. Also, the axial has the ability to increase its pressure ratio by adding extra stages. But centrifugals are still used in smaller engines which are simple and rugged.
Since high pressuer ratios is a favourite, so axials are chosen because of the improved efficiency, which in turn leads to improved specific fuel consumption for a given thrust.
Gravity explains the motions of the planets, but it cannot explain who set the planets in motion.
EconoBoy From United Kingdom, joined Mar 2004, 157 posts, RR: 0
Reply 10, posted (10 years 1 month 1 week 5 days 1 hour ago) and read 32767 times:
An analogy to describe the differences in axial v centrifugal is to imagine pushing water up a slope with a broom. An axial compressor does it in a series of small, short sweeps whereas a centrifugal does it in one big shove. The biggest disadvantage of a centrifugal engine is its portly diameter.
Francoflier From France, joined Oct 2001, 3762 posts, RR: 11
Reply 13, posted (10 years 1 month 5 days 17 hours ago) and read 32767 times:
The Garret engines (TPE 331) use a set of 2 centrifugal compressors.
The centrifugal compressor allow for a greater compression ratio per compressor stages, i.e. one centrifugal compressor wheel get a certain amount of air to a higher pressure than one axial compressor wheel could.
It is especially useful on small engines, like the turboprop engines. It saves space and weight. Ever seen how small a TPE 331 is?
Say about 2 and 1/2 times the lenght of the screen in front of you. And it puts out more than 1000 shp...
Looks like I picked the wrong week to quit posting...